1. Field of the Invention
The invention relates to magnetic azimuth detectors, otherwise known as flux valves, particularly with respect to error compensation apparatus therefor.
2. Description of the Prior Art
Gyromagnetic compass systems utilizing a directional gyroscope slaved to a magnetic azimuth detector or flux valve have long been utilized in aircraft to provide azimuthal data. As is known, the flux valve is subject to errors such as index error as well as one and two cycle errors caused, inter alia, by misalignment of the flux valve in the aircraft, by spurious magnetic fields present in the aircraft and by anomolies of the electrical transmission systems utilized in transmitting the flux valve data. Generally misalignment or index error is eliminated by careful installation of the flux valve in the aircraft or by utilizing a pre-indexed flux valve which is precisely mounted on an installation plate by the device manufacturer, the plate being provided with a reference mark that establishes the reference magnetic axis of the valve. Alternatively, electrical compensation may be utilized to correct the index error in a well known manner.
An early prior art procedure for calibrating and compensating the compass system entailed "swinging the compass" with respect to a "compass rose". The compass rose is a pattern of radial lines painted on a concrete surface located in a generally remote magnetically stable area in the vicinity of an airport, the compass rose lines being precisely oriented along the cardinal and inter-cardinal headings. Prior art compass rose patterns typically utilize as many as 24 calibrator headings. Swinging the compass comprises orienting the aircraft precisely with respect to the plurality of compass rose reference lines with respect to which flux valve errors are determined. Utilizing charts or mathematical formulas, compensation values are set into potentiometers to reduce the flux valve errors. It is appreciated that such a procedure is necessarily exceedingly time consuming as well as requiring sizable trained ground crews both for precisely orienting the aircraft along the heading lines and for performing the compensation adjustments once the errors are determined. Such procedure additionally requires accurately surveyed swing sites.
Other prior art apparatus and procedures have been developed for calibrating the gyromagnetic compass system of an aircraft. These methods entail positioning the aircraft in a magnetically stable area and measuring the earth's magnetic field at the location of the flux valve utilizing external field measuring equipment. The compass calibration apparatus included means for injecting currents into the flux valve so as to cancel the earth's field therein in accordance with its measured value and thereafter applying currents to the valve so as to generate precise fields simulating the earth's magnetic field at the desired cardinal and inter-cardinal calibration headings. Flux valve errors measured with respect to these artificially generated "electrical swinging" fields are utilized to adjust the compensation potentiometers so as to reduce the errors. This prior art field cancellation and electrical swinging procedure requires significant amounts of ground equipment as well as sizable ground crews to perform the procedures. Considerable time is also required for these complex compass calibration processes.
Another prior art approach to compass system calibration is the two heading random swing. In this procedure the aircraft is approximately aligned along a randomly oriented line at a magnetically stable area and the corresponding flux valve output is recorded. The misalignment of the aircraft with respect to the heading line is measured by the ground crew and entered into the system. The aircraft is then reoriented to approximate alignment along the line in the opposite direction with the flux valve output and misalignment data again obtained. With these data a measure of the earth's field is derived and utilized in providing an earth's field cancelling current to the flux valve in a manner generally similar to that described above. Thereafter an electrical swing of the type described is performed in order to obtain the errors utilized for compensation. It is appreciated that the two heading random swing is also undesirably time consuming, requires a trained ground crew and utilizes complex and expensive ground support equipment.
Thus it is appreciated that the magnetic azimuth detector is an integral part of gyromagnetic compass systems utilized in aircraft. Mounting of the detector in an aircraft, which has numerous ferromagnetic components that distort the earth's field, induces one and two cycle errors in the flux valve output. As described above, prior art compensation of these errors has involved calibration procedures during which ground equipment is connected to the aircraft, specially trained personnel thereafter performing the calibration by manually rotating the aircraft or electrically rotating the flux valve and adjusting the potentiometers on the equipment. The conventional manual compass swings required special equipment, such as plum bobs and alignment equipment, and additionally were time consuming. Although the prior art electrical swings could meet accuracy requirements, such procedures required considerable test equipment and highly trained operators to achieve satisfactory results.